1. Field of the Invention
The present invention relates generally to an electrographic apparatus and more specifically to an improved structural arrangement in an electrographic apparatus of the type having a resistive charging device with scorotron characteristics, which arrangement achieves generation of a corona.
2. Description of the Prior Art
In electrostatographic applications such as in xerography, a charge retentive surface is electrostatically charged, and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The remaining pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto, thus forming a permanent record of the image to be reproduced. The process is well known, and is useful for light lens copying from an original, and printing applications from electronically generated or stored originals. The process has analogs in other electrostatographic applications such as, for example, ionographic applications, where charge is deposited on a charge retentive surface in accordance with an image stored in an electronic form.
It is a common practice in electrophotography and its analogs to use wire corona generating devices to provide electrostatic fields driving various machine operations. Thus, corona devices are used to deposit charge on the charge retentive surface prior to exposure to light, to implement toner transfer from the charge retentive surface to the substrate, to neutralize charge on the substrate for removal from the charge retentive surface, and to clean the charge retentive surface after toner has been transferred to the substrate. These corona devices normally incorporate at least one fine wire coronode held at a high voltage to generate ions or charging current to charge a surface closely adjacent to the device to a uniform voltage potential, and may contain screens and other auxiliary coronodes to regulate the charging current or control the uniformity of charge deposited. The devices may be driven with positive or negative D.C. potentials.
Dicorotrons are A.C. driven corona devices incorporating a dielectric coating over the active coronode structure. Thus, dicorotrons provide an arrangement, which has the characteristics of an array of capacitors coupled to the air between the voltage source and the charge retentive surface. This arrangement blocks D.C. current flow from the coronode to the charge retentive surface. The charging current to the charge retentive surface at any point on the coronode surface is limited by the maximum displacement current that can be delivered by the dielectric, which provides an essentially self regulating device. Applying a large A.C. potential to the coronode creates a gaseous plasma at the coronode that is maintained by the displacement current at any point. If the plasma current exceeds the displacement current at any point along the coronode, such as in the case of a non-uniformity caused by dust or debris on the coronode, the plasma potential drops and the plasma current is quenched at that point. If the plasma current is too low, the plasma potential rises and the plasma current is forced to increase. As a result of this action, the overall discharge from the coronode to the surface tends to be uniform since each point on the coronode surface delivers the same net charge per unit area to the plasma as every other point during each voltage reversal. Any current extracted from the plasma to charge the charge retentive surface is therefore uniform and the device tends to be stable because of its self regulating behavior.
By contrast, D.C. driven bare wire devices such as corotrons have a tendency to arc at non-uniformities along the coronode which, in effect, causes each point along the coronode to compete for current at the expense of adjacent areas. Thus, non-uniformity has an effect of reducing the available corona current along the entire coronode. This effect has a tendency to be more pronounced in negative charging devices.
Another technique employed, commonly referred to as a "scorotron," is described in U.S. Pat. No. 2,777,957. The main basis for the scorotrons design is to limit the charge potential on a surface by using a wire grid or screen placed between the corona discharge wire and the surface. The grid is maintained at a predetermined potential and serves to terminate further charging of the surface when the surface potential on all portions of the surface being charged corresponds to the grid potential. The grid can be grounded or biased by means of an external voltage source, or it can be self-biased from the corona current by connecting the grid to a ground arrangement through current flow restricting devices (an example of the latter being illustrated in U.S. Pat. No. 3,729,649). Superior image reproductions are obtainable only when very uniform electrostatic charges are established on the surface before imaging.
Thus far it has been demonstrated that it is known in the copying art to use corotrons, di-corotrons, scorotrons or other similar corona generating devices. For example, U.S. Pat. No. 4,879,569 discloses a charge generating device having improved reliability and predictability of electrical discharge. It further discloses to provide a print cartridge for use in a charge transfer imaging having improved reliability and predictability of image creation. This is done by providing a print cartridge with increased efficiency by directing a greater portion of the charged particles created towards the image receiving surface. That patent discloses an arrangement of laminates comprised of a dielectric material, such as glass reinforced epoxy. The underside of the dielectric material is comprised of printed driver electrodes. The driver electrodes are placed parallel to finger electrodes, and the types of electrodes are separated by a mica strip. The arrangement provides improved reliability and predictability of an electrical discharge. It also yields increased efficiency, such that a greater portion of charged particles are directed towards an image receiving surface.
U.S. Pat. No. 4,811,158 discloses the idea of using a solid state charger having covered electrodes with dielectric and uncovered discharge electrodes disposed parallel to each other. The covered electrodes and the discharge electrodes are both disposed on the same surface of an insulating substrate. An AC voltage is applied between the covered electrodes and the discharge electrodes, or between the covered electrodes, and a DC voltage is applied to the discharge electrodes for charging photosensitive members in electrostatic recorders.
U.S. Pat. No. 4,783,716, discloses an apparatus for electrically discharging or charging a member to be discharged or charged. The device includes a dielectric member, first and second electrodes embedded in the dielectric member. The first and second electrodes being supplied with an alternating voltage therebetween to cause discharge of an adjacent part of a surface of the dielectric member at a predetermined discharge starting voltage. A third electrode, disposed to or adjacent a part of the surface of the dielectric member at such a position as when the discharge occurs by application of the alternating voltage between the first and second electrodes, no discharge occurs between the first electrode and the third electrode or between the second electrode and the third electrode.
Another example is U.S. Pat. No. 4,779,107, which discloses a marking array for use in an ionographic marking apparatus in which the ion modulation structure is subject to a lightly corrosive atmosphere. Improved marking electrodes are provided which comprise a thin film body of a conductive material having a surface which is chemically neutral to the corona effluents.
U.S. Pat. No. 4,562,447 discloses the use of an ion modulating electrode having at least one row of a plurality of apertures, which is capable of enhancing or blocking the passage of an ion flow through said apertures. The ion modulating electrode comprises a continuous layer of a conductive material and a segmented layer of a conductive material separated from each other by an insulating layer. The ion modulating electrode comprises at least one of said continuous layers and the segmented layer has a resistance layer.
The current invention is an improvement over U.S. Pat. No. 4,794,254, having common inventors and assignee as the present application, which is herein incorporated by reference in whole. That patent discloses a distributed resistance corona charging device. In particular, this corona charging device included an insulating substrate, a resistive material layer deposited on the substrate to a uniform thickness, and a plasma gap separating the resistive material layer into at least two resistive material regions. Voltage is applied to the resistive material regions through electrodes arranged on the resistive material regions so that a uniform resistance is provided between the power supply and the points on the resistive material regions immediately adjacent to the plasma gap. The distributed resistance corona generating device is inherently self regulating providing a uniform charging potential along the gap. Most notedly, this device does not use a metal wire corona electrode, and the device works like a corotron, including the inherent problems associated with corotron-type devices (such as lacking the ability to terminate charging of the surface once enough charge is built up).
Although these current corona producers (corotron, scorotron, dicorotron, and di-scorotron) perform satisfactorily, over prolonged use, say for example 150,000 cycles, difficulties are experienced for thin metal wire corona electrodes. These difficulties take the form of undeveloped streaks being formed in the copies produced resulting in unpredictable images.
As a result, the present invention provides a solution to the described problems and other problems, and also offers other advantages over the prior art.